1. Field of the Invention:
This invention relates to a luminance signal and chrominance signal separating circuit for effectively separating a luminance signal and chrominance signal from a composite video signal, for example, in a color television receiving set. Description of the Related Art:
In an NTSC system which is one of color television signal transmitting systems, primary color signals of red, green and blue are remade into a luminance signal representing the brightness of the picture surface and a chrominance signal having information of a hue and chroma and a composite video signal superimposing both of these signals is transmitted. This is based on the idea that, as a luminance signal is already transmitted for a black and white television receiving set, in order to reproduce a correct color in a color television receiving set, it is necessary to transmit a chrominance signal which will become a primary color signal when added with a luminance signal.
The relation between the color subcarrier for transmitting the chrominance signal and the luminance signal in the above mentioned composite video signal is set to be as shown in FIG. 10 so that no obstruction may be produced between the signals with each other. That is to say, a chrominance signal (C) having 3.57954 MH.sub.z as of a subcarrier is inserted into a high band corresponding part of a luminance signal (y) of 0 to 4.2 MH.sub.z. Therefore, in order to reproduce a broadcasting signal with a color television receiving set, it is necessary to separate the luminance signal and chrominance signal from each other. There are usually used such two methods of separating the luminance signal and chrominance signal from each other as in the following.
The first is a method by using a trap wherein a composite video signal is passed through a trap of 3.58 MH.sub.z so that the chrominance signal may be absorbed and only the luminance signal may be taken out and the composite video signal is passed through a band pass filter of 3.58 MH.sub.z .+-.0.5 MH.sub.z so that the chrominance signal may be taken out.
The second is a method by a comb filter wherein it is utilized that, in an NTSC system, the phase of a color subcarrier is inverted (to be an inverted phase) every horizontal scanning period (which shall be called 1H hereinafter). FIG. 11 shows the formation of a comb filter. In FIG. 12, (a) to (d) show waveforms at respective points (a) to (d) in FIG. 11. However, in FIG. 12, the composite video signal in 1H is represented by a combination of a luminance signal (y) and chrominance signal (C) but the synchronizing signal is omitted for the sake of brevity.
In FIG. 11, the reference numeral 1 represents an input terminal of a composite video signal. The composite video signal input into the terminal 1 is delayed by 1H by a 1H delaying circuit 2 and is added to an adding circuit 3 in which the signal (of the waveform (a)) before being delayed and the signal (of the waveform (b)) delayed by 1H are added together. By this addition, the chrominance signal is canceled and the luminance signal component is taken out. However, the luminance signal component becomes twice as high in the level and is therefore attenuated to be on a 1/2 level by an attenuator 7 to obtain a luminance signal (y) from an output terminal 4. Also, the composite video signal input into the terminal 1 is delayed by 1H by the 1H delay circuit 2 and is added to a subtracting circuit 5 in which the signal (of the waveform (a)) before being delayed is subtracted from the signal (of the waveform (b)) delayed by 1H. By this subtraction, the luminance signal is canceled and the chrominance signal component is taken out. However, the chrominance signal component becomes twice as high in the level and is therefore attenuated to be on a 1/2 level to obtain a chrominance signal (C) from an output terminal 6.
However, in such conventional luminance signal and chrominance signal separating circuit using the trap or combe filter as is described above, such problems as in the following will occur.
That is to say, when the first method by the above mentioned trap is used, simultaneously with removing the chrominance signal, the high band component of the luminance signal (concretely the component of 3.58 MH.sub.z of the luminance signal) will be also removed, therefore fine pictures will become obscure and the picture quality will reduce.
When the second method by the above mentioned comb filter is used, as shown in FIG. 13(a), in case there are parts having no correlation before and after the 1H of the composite video signal, on the part having a correlation between the composite video signal shown in (a) and the 1H delayed composite video signal shown in FIG. 13(b), after the addition, without removing the high band component (which component is not inverted in the phase in each 1H as different from the chrominance signal) of the luminance signal as in the part A in FIG. 13(c), the chrominance signal will be able to be removed. However, on the parts B and C having no correlation before and after the 1H, the chrominance signal component (represented by the reference symbol e in the drawing) will remain in the luminance signal and dots will be produced in the edge part of the picture image to the disadvantage. Also, the luminance signal itself will be of an amplitude 1/2 (represented by the reference symbol f in the drawing) as large as in the part A in the parts B and C and the reproduced picture image will be a dull picture image having an expansion in the vertical direction.
FIG. 13(d) shows a chrominance signal by a subtraction. Still, in the parts B and C, the luminance signal component (represented by the reference symbol g in the drawing) will remain in the chrominance signal, a cross color will be produced in the edge part of the picture image to the disadvantage, the same as in the luminance signal, the chrominance signal itselt will be also of an amplitude 1/2 (represented by the reference symbol h in the drawing) as large as in the part A and the reproduced picture image will be a dull picture image having an expansion in the vertical direction.
Therefore, in order to solve the above mentioned defects, the present applicant has suggested a luminance signal and chrominance signal separating circuit by a comb filter (mentioned in a Japanese patent application No.154464 filed on June 21, 1988) wherein, even in case there are parts having no correlation before and after 1H of a composite video signal, it will be possible to effectively separate a luminance signal and chrominance signal from a composite video signal.
Now, the band of this suggested comb filter depends on the band width of a band pass filter (BPF) near 3.58 MH.sub.z. However, in case the comb filter band is taken to be wide, the influence of the luminance component (of a band of 3.58 MH.sub.z) will be so large that no sufficient effect will be obtained in some case. That is to say, in case a video image of such large luminance variation as is shown in FIG. 14(a) is displayed on the picture surface, the video signal of its horizontal scanning line will have a chrominance signal C (a blue signal in the drawing) superimposed on the luminance signal varying largely as shown in FIG. 14(b). In such case, in the edge part of the luminance signal, the luminance signal and chrominance signal will simultaneously vary, the luminance signal component of the band of 3.58 MH.sub.z and the chrominance signal will simultaneously pass through the BPF, the component of the band of 3.58 MH.sub.z of the luminance signal will not be separated but will remain in the boundary part of the color and the chrominance signal component will not be separated but will remain in the edge part of the luminance signal.
As in the above, in the conventional luminance signal and chrominance signal separating circuit, in case a trap is used, there has been a problem that, when the chrominance signal is removed, the high band component of the luminance signal will be also removed. Also, in case a comb filter is used, there has been a problem that, when there are parts having no correlation before and after the 1H of the composite video signal, the luminance signal and chrominance signal will not be able to be perfectly separated or, in the edge part of a video image of a large luminance variation, the chrominance signal component will mix into the separated luminance signal or the luminance signal component will mix into the chrominance signal.